monster.distance.clj

(ns monster.distance)

(defn min-monster-distance
  "Given a collection of monster positions `positions` (numbers) and a minimum
  safe separation `d`, return

  * the smallest distance between any two monsters **if** that distance
    is less than `d`
  * -1 if every pair of monsters is at least `d` units apart.

  Runs in O(n log n) time (due to sorting) and O(n) space."
  [positions d]
  (let [sorted     (sort positions)
        ;; pair-wise distances between consecutive sorted positions
        distances  (map - (rest sorted) sorted)
        ;; handle zero- or one-monster edge cases
        min-dist   (when (seq distances) (apply min distances))]
    (if (or (nil? min-dist)               ; 0 or 1 monster
            (>= min-dist d))              ; all distances safe
      -1
      min-dist)))